The present invention relates to electrographic copying or printing systems, and more particularly to the fusing of toned images in such systems on a recording member. In such electrographic printing systems, a drum or belt is made to carry an electrostatic latent image, and the image is developed by a toner to form a visible image that is transferred to an ultimate recording member such as a paper or plastic sheet.
In a number of systems, the toner image is transferred to the sheet by means of a counter-electrode charged to provide electrostatic attraction; in others, it may be transferred by contact pressure. In either case, the transferred image remains relatively unconsolidated and loosely bound to the sheet, so that fusing is generally required to render the image stable and resistant to smearing.
For this purpose the image-bearing sheet is carried to another station within the printer at which heat, pressure or the like is applied to fuse the toned image, thus consolidating and adhering it permanently to the sheet. Such heat fusing may be effected by contacting the sheet to a heated platen or hot roll, or by passing it under a radiant heater or flash source of radiant energy. In some systems of this type, an intermediate transport member such as a separate belt is used to pick up the toned image from one dielectric imaging surface and transfer the toned image to a sheet which is brought into contact with the transfer member at the nip of a transfer roller. This transfer belt is heated, for example, by passing over a heated roller. In the latter case, the toned image is transferred and is simultaneously at least partially fused to the sheet as it passes the nip of the transfer roller. However, even in a hot-transfer construction, a post-fuser station is often desirable to achieve image permanence and suitable surface finish characteristics.
In systems involving electrostatic image formation with toning of the latent image, and transfer of the toned image to a sheet or ultimate recording member, the range of constructions is constrained, in that separate mechanisms are generally provided for image transfer and for image fusing, or in that when both events occur simultaneously, this is done by a heated transfer roller that contacts each portion of the image for a very brief time. In these systems the fusing station is generally maintained at a temperature substantially above the toner melting point, and significant amounts of heat energy may be lost from the fusing station to the transfer member. While such instantaneous-contact constructions maintain high sheet feed rates the high temperature fuser may limit the energy efficiency of the fusing step, and introduce further complexity when used with a variety of different toners.
It is therefore desirable to provide a printing process of higher thermal efficiency, or one that is carried out at lower temperatures, and which operates well with a variety of different toners.